TETRAHEDRAL-SITE OCCUPANCIES IN SODIUM ALUMINUM-GALLIUM FELDSPAR SOLID-SOLUTIONS [NA(AL1-XGAX)SI3O8]

The structures of two sodium aluminum-gallium feldspars [Na(Al1-xGax)Si3O8] have been synthesized hydrothermally, and refined at room temperature with single-crystal X-ray intensities. Tetrahedral-site occupancies for gallium have been obtained by direct refinement and for aluminum by analysis of partial T-O distances (T =Si, Al, Ga). For x = 0.1, 630°C, 1.3 kbar, R = 0.031, gallium occupancies for the four tetrahedral positions are t10 = 0.070, t1m = 0.000(1), t20 = 0.014(1), t2m = 0.014(1), and aluminum occupancies are 0.35, 0.19, 0.18, 0.18, respectively. For x = 0.3, 631°C, 1.3 kbar, R = 0.032, gallium occupancies are t10 = 0.168, t1m = 0.016(1), t20 = 0.053(1), t2m = 0.063(1), and aluminum occupancies are 0.27, 0.13, 0.14, 0.15, respectively. The site preference of gallium is not significantly modified by change in the extended feldspar structure; gallium remains largely ordered with the site preference sequence t10 ≫ t20 ≈ t2m > t1m → 0.0. It does not appear that the ordering of gallium is influenced to any significant degree at the experimental temperatures by coupling with the lattice strain of the disordered host crystal. © 1991 Academic Press, Inc.